How to flip camera view?
3D  view/Render
Give the camera a negative scale on the local X axis.
Both S X X -1 and ⎈ CtrlM X do this (use Y instead of X for flipping vertically), or you can set the scale directly in the transform panel:

Rendered animation playback
When previewing an already-rendered animation (⎈ CtrlF11) you can flip the display horizontally with F and vertically with Shift-F.
Sequencer
The VSE has an option to flip strips in Properties region &gt; Filter:


Thanks ideasman42 for mentioning these :)